Open Channel Flow Dimensional Analysis.pdf

Open Channel Flow Dimensional Analysis

1. DIMENSIONAL ANALYSIS
(OPEN CHANNEL FLOW AND HYDRAULIC
MACHINERY)
UNIT – II
Rambabu Palaka, Assistant Professor BVRIT

2. Learning Objectives
1.Introduction to Dimensions & Units
2.Use of Dimensional Analysis
3.Dimensional Homogeneity
4.Methods of Dimensional Analysis
5.Rayleigh’s Method

3. Learning Objectives
6.Buckingham’s Method
7.Model Analysis
8.Similitude
9.Model Laws or Similarity Laws
10.Model and Prototype Relations

4. Many practical real flow problems in fluid mechanics can be solved by using
equations and analytical procedures. However, solutions of some real flow
problems depend heavily on experimental data.

Sometimes, the experimental work in the laboratory is not only time-consuming,
but also expensive. So, the main goal is to extract maximum information from
fewest experiments.

In this regard, dimensional analysis is an important tool that helps in correlating
analytical results with experimental data and to predict the prototype behavior from
the measurements on the model.
Introduction

5. Dimensions and Units
In dimensional analysis we are only concerned with the nature of the
dimension i.e. its quality not its quantity.

Dimensions are properties which can be measured.
Ex.: Mass, Length, Time etc.,
Units are the standard elements we use to quantify these dimensions.
Ex.: Kg, Metre, Seconds etc.,

The following are the Fundamental Dimensions (MLT)
Mass kg M
Length m L
Time s T

6. Secondary or Derived Dimensions
Secondary dimensions are those quantities which posses more than one
fundamental dimensions.
1. Geometric
a)Area
b)Volume
m
2
m
3
L
2
L
3
2. Kinematic
a)Velocity
b)Acceleration
m/s
m/s
2
L/T
L/T
2
L.T
-1
L.T
-2
3. Dynamic
a)Force
b)Density
N ML/T
kg/m
3
M/L
3
M.L.T
-1
M.L
-3

7. Problems
Find Dimensions for the following:
1.Stress / Pressure
2.Work
3.Power
4.Kinetic Energy
5.Dynamic Viscosity
6.Kinematic Viscosity
7.Surface Tension
8.Angular Velocity
9.Momentum
10.Torque

8. Use of Dimensional Analysis
1.Conversion from one dimensional unit to another
2.Checking units of equations (Dimensional
Homogeneity)
3.Defining dimensionless relationship using
a)Rayleigh’s Method
b)Buckingham’s π-Theorem
4.Model Analysis

9. Dimensional Homogeneity
Dimensional Homogeneity means the dimensions in each
equation on both sides equal.

10. Problems
Check Dimensional Homogeneity of the following:
1.Q = AV
2.E
K = v
2/2g

11. Rayeligh’s Method
To define relationship among variables

This method is used for determining the
expression for a variable which depends upon
maximum three or four variables only.

12. Rayeligh’s Method
To define relationship among variables

This method is used for determining the
expression for a variable which depends upon
maximum three or four variables only.
Methodology:

Let X is a function of X
1 ,X
2, X
3 and mathematically it can be written as
X = f(X
1, X
2, X
3)

This can be also written as
X = K (X
1
a
, X
2
b
, X
3
c
) where K is constant and a, b and c are arbitrarily powers

The values of a, b and c are obtained by comparing the powers of the fundamental
dimension on both sides.

13. Rayeligh’s Method
To define relationship among variables

This method is used for determining the
expression for a variable which depends upon
maximum three or four variables only.
Methodology:

Let X is a function of X
1 ,X
2, X
3 and mathematically it can be written as
X = f(X
1, X
2, X
3)

This can be also written as
X = K (X
1
a
, X
2
b
, X
3
c
) where K is constant and a, b and c are arbitrarily powers

The values of a, b and c are obtained by comparing the powers of the fundamental
dimension on both sides.
Problem: Find the expression for Discharge Q in a open channel flow when Q is
depends on Area A and Velocity V.

Solution:
Q = K.A
a
.V
b
 1
where K is a Non-dimensional constant
Substitute the dimensions on both sides of equation 1
M
0 L
3 T
-1
= K. (L
2
)
a
.(LT-
1
)
b
Equating powers of M, L, T on both sides,
Power of T,
Power of L,
-1 = -b  b=1
3= 2a+b  2a = 2-b = 2-1 = 1
Substituting values of a, b, and c in Equation 1m
Q = K. A
1
. V
1
= V.A

14. Rayeligh’s Method
To define relationship among variables

This method is used for determining the
expression for a variable which depends upon
maximum three or four variables only.
Methodology:

Let X is a function of X
1 ,X
2, X
3 and mathematically it can be written as
X = f(X
1, X
2, X
3)

This can be also written as
X = K (X
1
a
, X
2
b
, X
3
c
) where K is constant and a, b and c are arbitrarily powers

The values of a, b and c are obtained by comparing the powers of the fundamental
dimension on both sides.

15. Rayeligh’s Method
To define relationship among variables

This method is used for determining the
expression for a variable which depends upon
maximum three or four variables only.
Methodology:

Let X is a function of X
1 ,X
2, X
3 and mathematically it can be written as
X = f(X
1, X
2, X
3)

This can be also written as
X = K (X
1
a
, X
2
b
, X
3
c
) where K is constant and a, b and c are arbitrarily powers

The values of a, b and c are obtained by comparing the powers of the fundamental
dimension on both sides.

16. This method of analysis is used when number of variables are more.

Theorem:
If there are n variables in a physical phenomenon and those n variables contain m dimensions,
then variables can be arranged into (n-m) dimensionless groups called Φ terms.

Explanation:
If f (X
1, X
2, X
3, ……… X
n) = 0 and variables can be expressed using m dimensions then
f (π
1, π
2, π
3, ……… π
n - m) = 0 where, π
1, π
2, π
3, … are dimensionless groups.
Each π term contains (m + 1) variables out of which m are of repeating type and one is of non-
repeating type.
Each π term being dimensionless, the dimensional homogeneity can be used to get each π term.

π denotes a non-dimensional parameter
Buckingham’s π-Theorem

17. Selecting Repeating Variables:
1.Avoid taking the quantity required as the repeating variable.
2.Repeating variables put together should not form dimensionless group.
3.No two repeating variables should have same dimensions.
4.Repeating variables can be selected from each of the following
properties.
Geometric property  Length, height, width, area
Flow property  Velocity, Acceleration, Discharge
Fluid property  Mass density, Viscosity, Surface tension
Buckingham’s π-Theorem

18. Example

19. Example

20. Example

21. For predicting the performance of the hydraulic structures (such as dams, spillways
etc.) or hydraulic machines (such as turbines, pumps etc.) before actually
constructing or manufacturing, models of the structures or machines are made and
tests are conducted on them to obtain the desired information.

Model is a small replica of the actual structure or machine
The actual structure or machine is called as Prototype
Models can be smaller or larger than the Prototype

Model Analysis is actually an experimental method of finding solutions of
complex flow problems.
Model Analysis

22. Similitude is defined as the similarity between the model and prototype in
every aspect, which means that the model and prototype have similar
properties.

Types of Similarities:
1.Geometric Similarity  Length, Breadth, Depth, Diameter, Area,
Volume etc.,
2.Kinematic Similarity  Velocity, Acceleration etc.,
3.Dynamic Similarity  Time, Discharge, Force, Pressure Intensity,
Torque, Power
Similitude or Similarities

23. The geometric similarity is said to be exist between the model and
prototype if the ratio of all corresponding linear dimensions in the model
and prototype are equal.
Geometric Similarity
L B D
r
m m m
 L
LP
 BP
 DP
r
2
L
AP
Am

r
3
L
VP
Vm

where Lr
is Scale Ratio

24. The kinematic similarity is said exist between model and prototype if the
ratios of velocity and acceleration at corresponding points in the model and
at the corresponding points in the prototype are the same.
Kinematic Similarity
VP
Vm
 Vr
aP
am
 ar
whereVr
is Velocity Ratio where ar
is Acceleration Ratio

Also the directions of the velocities in the model and prototype should be same

25. The dynamic similarity is said exist between model and prototype if the
ratios of corresponding forces acting at the corresponding points are
equal
Dynamic Similarity
FP
Fm
 Fr
where Fr
is Force Ratio
Also the directions of the velocities in the model and prototype should be same
It means for dynamic similarity between the model and prototype, the
dimensionless numbers should be same for model and prototype.

26. Types of Forces Acting on Moving Fluid
1. Inertia Force, F
i
It is the product of mass and acceleration of the flowing fluid and acts in the
direction opposite to the direction of acceleration.
It always exists in the fluid flow problems

27. Types of Forces Acting on Moving Fluid
1.Inertia Force, F
i
2.Viscous Force, F
v
It is equal to the product of shear stress due to viscosity and surface area of the
flow.

28. Types of Forces Acting on Moving Fluid
1.Inertia Force, F
i
2.Viscous Force, F
v
3.Gravity Force, F
g
It is equal to the product of mass and acceleration due to gravity of the flowing
fluid.

29. Types of Forces Acting on Moving Fluid
1.Inertia Force, F
i
2.Viscous Force, F
v
3.Gravity Force, F
g
4.Pressure Force, F
p
It is equal to the product of pressure intensity and cross sectional area of
flowing fluid

30. Types of Forces Acting on Moving Fluid
1.Inertia Force, F
i
2.Viscous Force, F
v
3.Gravity Force, F
g
4.Pressure Force, F
p
5.Surface Tension Force, F
s
It is equal to the product of surface tension and length of surface of the flowing

31. Types of Forces Acting on Moving Fluid
1.Inertia Force, F
i
2.Viscous Force, F
v
3.Gravity Force, F
g
4.Pressure Force, F
p
5.Surface Tension Force, F
s
6.Elastic Force, F
e
It is equal to the product of elastic stress and area of the flowing fluid

32. Dimensionless Numbers
V
Lg
InertiaForce
Gravity Force

Dimensionless numbers are obtained by dividing the inertia force by
viscous force or gravity force or pressure force or surface tension force or
elastic force.
1. Reynold’s number, R
e =
2.Froude’s number, F
e =

3.Euler’s number, E
u =
e
4. Weber’s number, W =
5. Mach’s number, M =
V
p / 
InertiaForce
PressureForce

InertiaForce V
SurfaceTensionForce  / L

  Viscous Force
Inertia Force

VL
or
VD
Inertia Force

V
Elastic Force C

33. 2 2
L V
T T
L L
 ρ
3 V
 ρ
2 L
V  ρ

37. The laws on which the models are designed for dynamic similarity are
called model laws or laws of similarity.
1. Reynold’s Model
Model Laws
Models based on Reynolds’s Number includes:
a)Pipe Flow
b)Resistance experienced by Sub-marines, airplanes, fully immersed bodies

38. The laws on which the models are designed for dynamic similarity are
called model laws or laws of similarity.
1.Reynold’s Model
2.Froude Model Law
Model Laws
Froude Model Law is applied in the following fluid flow problems:
a)Free Surface Flows such as Flow over spillways, Weirs, Sluices, Channels
etc.,
b)Flow of jet from an orifice or nozzle
c)Where waves are likely to formed on surface
d) Where fluids of different densities flow over one another

39. The laws on which the models are designed for dynamic similarity are
called model laws or laws of similarity.
1.Reynold’s Model
2.Froude Model Law
3.Euler Model Law
Model Laws
Euler Model Law is applied in the following cases:
a)Closed pipe in which case turbulence is fully developed so that viscous forces
are negligible and gravity force and surface tension is absent
b)Where phenomenon of cavitations takes place

40. The laws on which the models are designed for dynamic similarity are
called model laws or laws of similarity.
1.Reynold’s Model
2.Froude Model Law
3.Euler Model Law
4.Weber Model Law
Model Laws
Weber Model Law is applied in the following cases:
a)Capillary rise in narrow passages
b)Capillary movement of water in soil
c)Capillary waves in channels
d)Flow over weirs for small heads

41. Model Laws
The laws on which the models are designed for dynamic similarity are
called model laws or laws of similarity.
1.Reynold’s Model
2.Froude Model Law
3.Euler Model Law
4.Weber Model Law
5.Mach Model Law
Mach Model Law is applied in the following cases:
a)Flow of aero plane and projectile through air at supersonic speed ie., velocity
more than velocity of sound
b)Aero dynamic testing, c) Underwater testing of torpedoes, and
d) Water-hammer problems

42. If the viscous forces are predominant, the models are designed for dynamic
similarity based on Reynold’s number.
Reynold’s Model Law
 
p
P P
P
m
m m
m V L ρ V L ρ

V
L
t
r

r
r
 Time Scale Ratio 
Velocity, V = Length/Time  T = L/V
t
a
r
Vr
r
 Acceleration Scale Ratio 
Acceleration, a = Velocity/Time  L = V/T
    R R e e p m


45. Problems
1. Water flowing through a pipe of diameter 30 cm at a velocity of 4 m/s. Find the
velocity of oil flowing in another pipe of diameter 10cm, if the conditions of
dynamic similarity is satisfied between two pipes. The viscosity of water and oil
is given as 0.01 poise and 0.025 poise. The specific gravity of oil is 0.8.

46. If the gravity force is predominant, the models are designed for dynamic
similarity based on Froude number.
Froude Model Law
Lr Tr
 Scale Ratiofor Time 
g Lm g LP
m p
Vm

Vp
Lr Vr
 VelocityScale Ratio      F F e e p m

Tr
 Scale Ratio for Accele ration  1
2.5
Q
r
 Scale Ratio for Discharge  Lr
3
Fr
 Scale Ratio for Force  Lr
Fr
 Scale Ratio for Pressure Intensity  Lr
3.5
Pr
 Scale Ratio for Power  Lr

53. Problems
1.In 1 in 40 model of a spillway, the velocity and discharge are 2 m/s and 2.5
m
3
/s. Find corresponding velocity and discharge in the prototype

2.In a 1 in 20 model of stilling basin, the height of the jump in the model is
observed to be 0.20m. What is height of hydraulic jump in the prototype? If
energy dissipated in the model is 0.1kW, what is the corresponding value in
prototype?

3.A 7.2 m height and 15 m long spillway discharges 94 m
3
/s discharge under a
head of 2m. If a 1:9 scale model of this spillway is to be constructed,
determine the model dimensions, head over spillway model and the model
discharge. If model is experiences a force of 7500 N, determine force on the
prototype.

54. Problems
4.A Dam of 15 m long is to discharge water at the rate of 120 cumecs under a
head of 3 m. Design a model, if supply available in the laboratory is 50 lps

5.A 1:50 spillway model has a discharge of 1.5 cumecs. What is the
corresponding discharge in prototype?. If a flood phenomenon takes 6 hour to
occur in the prototype, how long it should take in the model

55. Reference
Chapter 12
A Textbook of Fluid Mechanics and
Hydraulic Machines
Dr. R. K. Bansal
Laxmi Publications